The protozoan parasite Giardia lamblia relies on a purine salvage pathway for its supply of purine ribonucleotides. An essential component of this pathway is the enzyme guanine phosophoribosyltransferase (GPRTase), which converts guanine into GMP. There is strong evidence that inhibiting GPRTase would disrupt purine salvage and kill the parasite. We propose to determine the 3-dimensional structure of GPRTase and use the resulting structural information to help us identify inhibitors of this enzyme.